In hindsight, it sounds obvious that the co-founders of Instagram would focus their engineering talent on getting photos to load quickly. Presumably that’s what users of Facebook’s photo-sharing app would want. But knowing where to focus your talent and what to ignore is harder than people think, said co-founder Mike Krieger.

Krieger said in the beginning, none of the Instagram team ever thought their company would get as big as it did, as fast as it did. “At the time we weren’t on any of Amazon’s services, we were just on this tiny server in L.A.”

But Kreiger said the team decided early on that their main priority would be to get photos loaded as quickly as possible so that users checking the app for a minute while waiting at a bus stop or on their commute would be instantly entertained. He didn’t want an app where the download took forever, or the photos wouldn’t appear.

“That minute has to be magical,” he said. “The part of the server that makes sure the photo gets to the app as quickly as possible, that’s what we focused on.”

This decision allowed Instagram to generally hold up to intense traffic, and Malik noted that the service almost never crashed. Kreiger said this is true, but there are also a few benefits of building for mobile, where users expect downtimes and delays.

“If it’s a two minute blip, some people will be like, ‘Oh, Instagram is down,’ but some people will be like ‘Oh, my phone sucks.'”

Check out the rest of our Mobilize 2012 coverage here, and the live stream can be found here.